Space debris is growing at a rapid rate and poses a hazard to future space vehicles. Although the probability of collision is low, collision speeds could be hypervelocity with the possibility of destruction of space structure.
Space debris can be conveniently grouped into three categories. The first is mission related debris. Inactive payloads, expended satellites, shrouds, clamps, and separation components are examples of debris within this category. A second category is launch related debris resulting from spent rocket stages, combustion products of the rocket stages (primarily aluminum oxide from solid propellants), and paint flakes. Unlike the first category, most of the debris within the second category is small and has less momentum than debris with greater mass. Nevertheless, the large oxide particles, at velocities of several kilometers per second, have the impact effect of a bullet, and enough force to destroy unprotected payloads. The third category of debris results from fragmentation. Collisions, explosion, breakups, and antisatellite (ASAT) testing contribute to this group. Sizes of debris in this group range from the very small untrackable objects to large, locatable objects.
Debris build-up patterns follow exploration of space and debris belts are forming along space trails. As documented in the literature, there is more debris in low earth orbit (LEO) as this was the first region explored and later used operationally. Additionally, all subsequent exploratory and operational missions go through this region, regardless of their final destination, leaving additional debris deposits. This, combined with the natural slow gravitational decay process which brings all the near-earth debris "home," makes the LEO the most debris littered region.
A second region of high interest is at geosynchronous altitude. A strategically placed payload at this altitude remains relatively fixed over a given position on earth. With minimal station keeping efforts and the need for only a single ground control station, a single satellite can provide constant coverage for almost half of the globe. This makes the geosynchronous altitude a prime parking site for communication and surveillance systems.
Many solutions for the elimination and/or control of space debris within the critical regions discussed are being proposed and have been developed. These solutions relate generally to structural hardening, avoidance, debris retrieval, earth reentry, transfer to dump regions, treaty/agreement, and collectors. Structural hardening has been a traditional solution to prevent penetration damage. This solution, however, has two shortcomings. First, space debris travels at hypervelocities (as high as 15 km/sec), well outside traditional hardening regimes. Technology for developing hardened structure does not exist at these velocities. Secondly, any hardening approach will most likely increase the system's total weight thereby increasingly launch cost and straining the capacity of current launch systems.
Avoidance schemes are being proposed in which orbital payloads would use thrusters to dodge debris on impact trajectories. These schemes, however, project tremendous fuel consumption, which for current satellites in unacceptable. Dodging debris would also require the development of sensor systems to detect and trace debris.
The debris retrieval concept has already been shown to be successful, at least in limited applications. U.S. Pat. No. 4,775,120 to Marwick describes an extraterrestrial transportation apparatus and method in which items could be crash transported to a low earth orbit crash-load capturing satellite for subsequent relocation. U.S. Pat. No. 4,750,692 to Howard relates to a satellite retrieval apparatus comprising a tethered grappling unit having deployable arms with catching ropes and Velcro hook strips on the ends thereof. On impacting the target, the catching ropes envelop the target and each other. The grappling unit is then retrieved along with the target satellite. In addition, a recent Shuttle mission retrieved an inactive satellite which was then repaired and returned to service. The drawback with this concept is the expense of retrieval and difficulty of the operation. Only high valued space debris would qualify for such a debris clearing solution.
Earth reentry involves using the last remaining onboard fuel to project the satellite into a decay orbit. One drawback with this concept is that energy needed to put a satellite into a decay orbit is significant, in many cases more than current satellites have onboard. The prior art provides several alternatives to the use of on-board fuel to project a satellite into a decay orbit. For example, U.S. Pat. No. 4,707,979 to Gutsche describes a method to produce and utilize propulsion forces on objects or devices by the controlled release of energy derived from absorbed radiation. U.S. Pat. No. 4,408,563 to Swales et al relates to a method of separating and ejecting a reentry body from a booster rocket. A pair of diametrically opposed rockets attached to the base of the re-entry body are ignited and provide thrust at an angle resulting in separation velocity and spin to the re-entry body. And, U.S. Pat. No. 3,427,808 to Butcher describes a method and apparatus to generate pressurized gas for satellite propulsion. A quantity of solid or liquid material which is decomposible into the gas state is provided, and either thermal decomposition, photolysis, or radiolysis are employed for the decomposition process. Even if future satellites were somehow required to carry a decay orbit propulsion reserve, this solution again is limited only to the payload category of debris and does little to mitigate the other two categories. Another drawback to this concept is the reentering payload impact location, which can have undesired political and safety implications.
Akin to the reentry concept is the concept of transfer of space debris to a dump orbit or region. Recognizing the burden placed on the propulsion system to cause reentry, especially for satellites in orbits other than the LEO, the concept is to move geosynchronous satellites from their strategic position when they become of little use. This concept also solves the impacting debris problem of the reentry solution, yet does little for the other categories of debris and raises the question of determining suitable junkyard regions.
Efforts to legislate away the space debris problem have had little success. Currently, some LEO regions have been set aside for space developing nations to use. Requirements of restricting solid rocket propulsion systems, eliminating blow away clamps, and similar requirements are being considered.
Overall, the existing and proposed solutions to eliminate or control space debris appear inadequate. None appear able to ensure safe, debris-free missions.
It is therefore the object of the present invention to provide a method to clear space debris to allow for safe orbits for spacecraft flight.
It is another object of the present invention to clear the space debris without resorting to expensive spacecraft to track, collect, and transport the debris.
It is another object of the present invention to clear space debris without creating additional debris in the form of solid by-products of detonation.